Software controllable circuit for resolver excitation switching in a motion control system

ABSTRACT

An auxiliary resolver position tracking (RPT) system for an industrial robot includes a resolver excitation and monitoring system which is powered by an uninterruptable power supply which includes a battery. The RPT system generates trapezoidal excitation pulses for the resolvers in the robot when no external excitation signal is applied, for example when the robot is shut down. Since there is relatively little motion to be detected in these instances, the RPT system switches between a slow position sampling rate, when no motion is detected, and a fast sampling rate when motion is detected. When operating on battery power, the RPT only switches to the fast sampling rate when motion is detected. To ensure that no motion data is lost when the system switches back to the slow sampling rate, the high sampling rate is maintained for a time sufficeint to capture any residual motion of the robot. In addition, the RPT includes circuitry to detect when the external excitation signal is absent, to detect when individual invalid position samples may be taken and to distinguish between valid motion and a jitter condition in which the sensed resolver position oscillates between two quadrants.

FIELD OF THE INVENTION

The present invention relates generally to a device for recording theposition reached by a moving part moved by a rotary motor. Inparticular, the present invention relates to a device which tracks theposition of a group of servomotors during power down conditions.

BACKGROUND OF THE INVENTION

Presently, in order to record the positions reached by a moving part(such as the multiple axis, moving arm of an industrial robot) it iscommon to use an angular-position transducer associated with the motorthat moves the part. The transducer, usually consisting of a resolver,generates a signal indicating the angular position reached by the motorduring a rotation. Since a typical industrial robot includes severalaxes of movement, it is desirable to use several resolvers so thatmovement may be monitored for each axis. The signal generated by theresolver associated with each axis of movement of the robot arm istransmitted to a general control unit (or servo controller SC) whichaccumulates the signal produced by each resolver (i.e. each axis ofmovement) to continually provide an absolute indication of the positionof the robot arm.

One of the main drawbacks of such a system occurs when the robot isseparated from its servo control (for example, in order to transfer itfrom the production premises to the user location or from one userlocation to another) or when the servo controller is momentarily powereddown (for example, following a power failure). In this instance, theinformation relating to the position reached by each moving part islost. In such a case, it may be necessary to completely recalibrate theposition tracking system before it may be restarted.

These loss of power situations are complicated by the fact that therobot arm continues to move for a few moments as a result of inertia,until it reaches a final stop position. This final stop position is notrecorded by the main position tracking system since the positiontracking system in the SC (as well as the robot arm and the resolvers)is no longer receiving operational power. Complicating manners stillfurther, is the possibility that the robot arm may be moved by externalforces during transport or by factory workers while powered down.

The effects of the above-mentioned drawbacks may be lessened, at leastin part, by using two cascade-connected angular transducers. Tworesolvers coupled together via a speed reduction mechanism is an exampleof such a device.

According to this configuration, the first resolver registers themomentary position reached during a rotation by the motor actuating themoving part, or the second resolver connected to the first resolver viaa speed reduction mechanism, such that the second resolver itselfperforms a single rotation during multiple rotations of the firstresolver. Using this configuration, the position of the moving part maybe "read" from the two resolvers when the SC is restarted.

This solution, know as a master/vernier or coarse/fine type, isimpractical both in terms of space (two resolvers and a speed reductionmechanism must be provided for each axis of movement of the robot) andin terms of cost (two resolvers with the associated precision gearingcost, in fact, more than a single resolver keyed onto the drive shaft).

Another solution is disclosed in U.S. Pat. No. 5,038,272 entitled"DEVICE FOR TRACKING AND RECORDING THE POSITION REACHED BY A MOVINGPART, IN PARTICULAR FOR INDUSTRIAL-ROBOT CONTROL", which is herebyincorporated by reference for its teaching on position tracking forindustrial robots.

In this system, an auxiliary resolver position tracking system isintegrally coupled to the industrial robot and powered by anuninterruptable power supply which includes, for example, a battery. Thesystem monitors the excitation signal applied to the resolvers and theoutput signals provided by the resolvers to maintain positioninformation for the various axes of rotation. When a failure in theexcitation signal is detected, the auxiliary system generates asubstitute excitation signal so that it may continues to monitor theoutput signals of the resolvers.

This system has disadvantages because the battery limits the time duringwhich it may monitor the position information returned by the resolvers.The resolver excitation signals are desirably provided at relativelyhigh current levels and, so, tend to drain the battery rapidly. The sizeof the battery which may be used in the resolver position trackingsystem is limited since it is desirable to package the position trackingsystem with the robot.

SUMMARY OF THE INVENTION

The present invention provides a device and method for recording theposition reached by a moving part moved by rotating shaft which, even ifthere is a failure in the external power supply, is able to retain theinformation relating to the position reached by the moving part of the arobot arm with respect to all of its possible axes of movement. Asidefrom having an auxiliary power supply to compensate for the loss of mainpower to the robot, the position tracking system of the presentinvention includes several power conservation systems which are operablewhen the auxiliary power supply is engaged, so that it may maintainposition information without using excessive battery current.

According to one aspect of the invention, when a failure in the externalpower supply occurs, this system of the present invention is activatedautomatically without any loss of resolver position information. Whilethe external power failure continues, in order to minimize the amount ofenergy used by the auxiliary power supply, the present invention has atleast two sampling rates for sampling the resolvers to determine whetherany motion has occurred. Thus, during periods of no motion during a mainpower failure, a relatively low frequency signal is used to sample theoutput signals of the resolver to minimize the use of an auxiliarypower. When motion is detected, however, a higher frequency samplingrate is activated to prevent any loss of position information.

According to another aspect, the present invention provides for avoidingfalse motion conditions such as "jitter" in detecting motion. Jitterdetection involves determining the present and two preceding positionsof the resolver. The two preceding positions are stored in a register.By comparing the three registers against a "look-up" table of possibleconditions, the system can detect whether actual motion has occurred orwhether a jitter condition, that is to say an oscillation betweenquadrants of the motor has occurred.

According to another aspect of the invention, the system continues toprovide a number of fast sampling pulses even after the fast samplingrate has been deactivated, to ensure that any motion occurring duringdeceleration of the part is adequately tracked. This provision ensuresthat the time between successive sampling pulses exceeds the possibletransition time of the resolver from one quadrant to another, the systemswitches to the slower sampling rate to preserve auxiliary power,without any loss of position information tracking.

According to another aspect of the invention, the system includes aswitching system which passes an excitation signal provided by anexternal source or an internally generated excitation signal responsiveto a control signal. The switching system generates the internalexcitation signal suitable for the resolver from a pulse signal. Theswitching system also controls the switching of the excitation signalsso that only one of the excitation signals is applied to the resolver atany one time.

According to another aspect of the invention, the system includesdetection circuitry which determines whether an open circuit conditionexists in the resolver windings. Upon detecting such an occurrence, thecircuitry flags an error condition.

According to yet another aspect of the invention, the system includescircuitry which monitors the external resolver excitation signal todetect intervals during which the excitation signal may not be valid.This circuitry is coupled to other circuitry in the system which ignoresposition information derived from the output signals of the resolversduring these intervals.

According to another aspect, the system includes circuitry whichmonitors the external periodic resolver excitation signal to determinewhen the excitation signal is valid or invalid. This circuitry comparesthe amplitude of the excitation signal to a predetermined thresholdvalue to produce an output signal which is in a first state when theamplitude is greater than the threshold and in a second state otherwise.The circuitry monitors the output signal for N periods of the excitationsignal to determine if the excitation signal is valid.

BRIEF DESCRIPTION OF THE FIGURES

The invention will now be described by way of a non-limiting example,with reference to the attached drawings in which:

FIG. 1a shows an example robot incorporating the present invention.

FIG. 1b shows a block diagram of the connections between the resolvers,the resolver position tracker and the servo controller.

FIG. 2 is a system block diagram of a resolver position tracker whichincludes an embodiment of the present invention.

FIGS. 3a and 3b are illustrations of servomotor motion and a jittercondition as detected by the resolver, respectively.

FIGS. 3c and 3d are timing diagram which are useful for describing thedifferent sampling rates used by the resolver position tracker shown inFIG. 2.

FIG. 4 is a flow-chart diagram for an exemplary program which controlsthe microcontroller shown in FIG. 2 to detect the jitter condition.

FIG. 5 is a flow diagram for an exemplary program which controls themicrocontroller shown in FIG. 2 to determine when the present inventioncan switch from a fast sampling rate to a slow sampling rate.

FIG. 6 is a block diagram, partly in logic diagram form, which showsdetails of the detection circuit of the resolver position trackingsystem shown in FIG. 2.

FIG. 7 is a block diagram, partly in schematic diagram form, which showsdetails of the excitation signal switch of the resolver positiontracking system shown in FIG. 2.

FIG. 8 is a block diagram, partly in schematic diagram form, which showsdetails of the magnitude detector circuit shown in FIG. 2.

FIG. 9 is a block diagram, partly in schematic diagram form, which showsdetails of the clock generator and valid servo data detector circuitsshown in FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1a shows an exemplary robot 110 controlled by servo controller 116.Servo controller 116 applies a sinusoidal excitation signal to resolvers112, coupled to the robot arm 110 and receives position information forthe robot arm 110 from the resolvers 112. Resolvers 112 are connected toservo controller through resolver position tracker (RPT) 114. In FIG.1a, robot 110 and resolvers 112 are being powered by main external powersupply (not shown). At the same time, RPT 114 maintains its owninternal, auxiliary power supply (not shown) in case of main powerfailure.

Robot 110 may have several axes of movement, the positions of which aretracked by the servo controller 116. In the present examples, robot 110has eight axes of movement. Consequently, eight resolvers (i.e. one foreach axis of movement) are positioned on their corresponding axes inrobot 110. FIG. 1b is a block diagram which illustrates the connectionof the resolvers 112 (consisting of resolvers 112a-112h) through RPT 114to servo controller 116.

During normal operation, when there is no power failure or shut down,the servo controller applies a single sinusoidal excitation signal toall of the resolvers 112 and receives two sets of output signals fromeach resolver. The excitation signal passes through the RPT before it isapplied to the resolvers. Signals from the resolvers are applied to theservo controller 116 and the RpT 114 in parallel. In fact, during normaloperation, the RPT is only detecting whether power is still present andmaintaining a table of resolver positions in case of main power failureor shut down.

In the exemplary embodiment of the invention, each resolver 112a-h maybe compared to a small rotating electrical machine consisting of a rotorand of a stator having two windings that are separated by an angle of90°. Alternatively, the machine may include two windings separated by anangle of 90° on the rotor and one winding on the stator. Each resolvermay be coupled, for example, to the shaft of a servomotor which in turndrives the robot on one of its axes of movement. Since each axis ofmovement of robot 110 is controlled by an individual motor, each motorhas a dedicated resolver.

Using the sinusoidal excitation voltage applied to the terminals of therotor winding of the motor, it is possible to obtain from the two statorwindings respective sinusoidal signals which are separated in phase by90° (i.e. a sine signal and a cosine signal). Taken together, thesesignals indicate the angular position of the rotor to a relatively highdegree of accuracy. The sine signal may be described as V_(IN) Sin O andthe cosine signal as V_(IN) Cos O, where V_(IN) is the sinusoidalexcitation signal and O is the instantaneous angular position of theresolver rotor with respect to a reference rotor position.

During normal operation of the robot, the servo controller 116 suppliesthe excitation signal to the resolver. An exemplary excitation signalmay be a sinusoidal signal having a frequency of approximately 5kilohertz (KHz) and an amplitude of approximately 6 volts peak to peak.These values are obviously intended as a guide only.

To be effective, the resolver and servo controller should be capable ofmonitoring the respective motors while they are rotating at theirmaximum speeds. Typical values of the motors used in the presentinvention are in the 3000-4800 RPM range. The acceleration of therespective motors must also be taken into consideration. The presentinvention, accelerations may be on the order of 800 quadrants/².

The sine and cosine signals produced by the resolver are sent to anangular position recording device (not shown) in the servo controller116. This device determines the angular position of the rotor of theresolver, and thus, the angular position of the corresponding axis ofmovement to a resolution of, for example, one arc-second.

The detection circuit 218 used in the exemplary RPT 114 is describedbelow with reference to FIG. 6. This circuit determines the angularposition of the resolver rotor with a resolution of only one quadrant.The servo controller 116 reads this position information only when it isrecovering from a power loss. This resolution is sufficient becauseservo controller 116 can readily determine the exact position of theresolver within a quadrant by exciting the resolver with a sinusoidalsignal and reading the resultant signals provided by the resolver.

Treating the algebraic sign of the sine and cosine signals as a binaryone (+) and a binary zero (-), a table can be constructed, such as table1, showing the quadrant in which the resolver is located. Thiscorresponds directly to the position of the particular axis of the robotcontrol arm.

                  TABLE                                                           ______________________________________                                        Quadrant     Angular range                                                                             Binary Code                                          ______________________________________                                        1             0°-90°                                                                     1,1                                                  2             90°-180°                                                                   0,1                                                  3            180°-270°                                                                   0,0                                                  4            270°-360°                                                                   1,0                                                  ______________________________________                                    

FIG. 2 is a block diagram of an exemplary resolver position tracker(RPT) 114. RPT 114 comprises micro-controller 220 which is the controlcenter for carrying out the functions associated with RPT 114. Connectedto micro-controller 220 is an excitation signal switch 210, a magnitudedetector 212, a valid servo data detector 214 and a clock generator 216.Also connected to micro-controller 220 is a position detection circuit218, a power supply 222, wave shape circuit 224 a wake-up timer 226, amemory (i.e. EEPROM) 228 and a communications port 230.

The microcontroller 220 used in the exemplary embodiment of theinvention is a HPC46003 device available from National Semiconductor.Alternatively, it is contemplated that any of a number ofmicrocontrollers or microprocessors may be used as the device 220. Thismicrocontroller includes 256 bytes of internal random access memory(RAM) storage. In addition, it includes an external memory port, coupledto an electrically erasable programmable read-only memory (EEPROM) 228,an interrupt port, coupled to a wakeup timer 226, a serial input/output(I/O) port coupled to an RS-485 communications port 230 and an internalclock signal generator (not shown) coupled to resonant crystal 232.

In the exemplary embodiment of the invention, the program which controlsthe operation of the microcontroller 220 is held in the EEPROM 228 whichalso holds data values that are updated relatively infrequently. Datavalues which are updated frequently are held in the internal RAM storagearea of the microcontroller.

The RPT system shown in FIG. 2 receives a sinusoidal servo excitationsignal from the servo controller 116. In the RPT 114, this signal isapplied to an excitation signal switch 210 which, in normal operations,passes the signal with minimal distortion to the resolvers 112 in theindustrial robot 110. When no external servo excitation signal isavailable, the excitation signal switch 210 generates an internalexcitation signal using a signal provided by the microcontroller 220 viaa waveshape circuit 224. The excitation signal switch 210 and waveshapecircuit 224 are described below in more detail with reference to FIG. 7.

The external excitation signal is applied to a magnitude detector 212,to a valid servo data detector 14 and to a clock generator 216. Themagnitude detector monitors the amplitude of the excitation signal inboth the positive and negative polarities and provides a signal to themicrocontroller 220. Using this signal, the microcontroller 220determines whether the excitation signal switch should pass the externalexcitation signal or produce the internal excitation signal. Themicrocontroller only switches between the external and internalexcitation signals when a trend of, for example, three valid cycles orthree invalid cycles of the external excitation signals has beendetected. This circuit is described in detail below with reference toFIG. 8.

The valid servo data detector also monitors the servo excitation signalbut to determine when distortion in the external excitation signal maycause individual samples from the resolvers 112 to be erroneous.Distortion of this type may be caused, for example by a rapidly varyingnon-linear load, such as a welding machine, in the electric powerdistribution system. This circuit is described in detail below withreference to FIG. 9.

The clock generator circuit 216 is also described in FIG. 9. Duringnormal operation, this circuit generates a SAMPLE CLOCK signal that islocked in phase to the sinusoidal servo excitation signal. When theservo excitation signal is not present, however, the clock generator 216switches the SAMPLE CLOCK signal to be a 1 KHz clock signal generated bythe microcontroller 220.

As described above, in response to the excitation signal, the resolvers112 produce sine and cosine signals which indicate the positions of therespective axes of motion for the robot 110. These signals are providedto the detection circuit 218 which analyzes the signals to develop anindication of the location of each axis. The microcontroller 220 pollsthe detection circuitry at a rate determined by a sampling clock signalto maintain a record of the position of each axis.

In addition to monitoring the position of each axis, the detectioncircuit 218 periodically monitors each of the resolver output signals todetect an open circuit condition in any of the resolver stator windings.Circuitry suitable for use as the detection circuit 218 is describedbelow with reference to FIG. 6.

A key feature of the exemplary resolver position tracking system is itsability to operate in a low-power mode when no external power isapplied, in order to conserve power in the battery. The power supply 22is an important factor in the implementation of this low-power mode. Thepower supply receives a 24 volt power signal from, for example, theservo controller 116. During normal operation, the power 222 supply usesthis 24 volt external power signal to generate all power signals used bythe RPT 114. In addition, the power supply 222 includes a battery 223,which is used to power the RPT 114 when the 24 volt signal is absent. Abattery charger 225 charges the battery during normal operatingconditions to ensure that the battery is available in the event of apower failure.

To further conserve power when the robot is still and the RPT isoperating from its battery 223, the RPT 114 enters its low-power mode.In this mode, only the microcontroller 220, clock generator and wakeuptimer are continuously powered. The remaining circuitry is only brieflypowered at intervals determined by a value that is set in the wake uptimer. The RPT 114 only enters its low-power mode when it determinesthat there is little or no motion on any of the axes of the robot 110.The low-power mode is described in detail below with reference to FIGS.3c, 3d and 5.

When the RPT is operating during a power outage or shutdown, it isdesirable for the resolvers to be excited by a signal from RPT 114. Thesignal that is used is a trapezoidal pulse generated from, a square-wavepulse provided by micro-controller 220. The trapezoidal pulseapproximates the analog wave form typically used to energize theresolvers from the windings. The trapezoidal pulse has sufficient slopeto operate the resolvers and provide the power ramp up and power rampdown similar to their normal excitation signal. The trapezoidal pulseput out by the RPT has a power level on the order of 6 watts(approximately 1 Ampere at 6 volts).

FIG. 6 is a block diagram of an exemplary detection circuit 218 whichincludes a resolver position detector and open circuit detector. Theresolver position detector produces an output signal from themultiplexer 626 which tracks the quadrant position of each of the eightresolvers. The open circuit detection system is designed to detect whenan open circuit condition exists in the windings of the resolver so thata error condition can be flagged. In response to this error condition,the RPT 114 ignores data from the defective resolver but continues tomonitor the other resolvers.

In the exemplary embodiment of the invention, 16 signals, where eachsignal includes a signal line and a return line, come into high-passfilters 610 and 612 from the eight resolvers 112. Each of the resolversin resolver set 112 provides two output signals (sine and cosinesignals). The purpose of the two output signals is to determine thequadrant location of the resolver in one of the four quadrants throughwhich the resolver can rotate.

The signal and return lines for each of the eight sine signals and eightcosine signals are applied to the high-pass filters 610 and 612.High-pass filter 610 and 612, which may have a -3 dB point of 200 Hz,are designed to eliminate any direct current (DC) components and tosubstantially attenuate any line frequency components (e.g. 50 Hz or 60Hz). Both the signal and return lines are processed through the highpass filters to ensure that no DC components are inadvertently added tothe sine and cosine signals.

From the high-pass filters, the signals pass through low-pass filters614 and 616. Low-pass filters 614 and 616 are also designed for noisereduction, more specifically to attenuate relatively high frequencyelectrical noise which may be generated by adjacent machinery andmotors. In the exemplary embodiment of the invention, these filters havea -3 dB point at approximately 7 KHz. Prior to entering low-passfilters, the eight signal lines are split off from each of the sine andcosine signals. These signals are applied to analog multiplexer 628 and630. Multiplexer 628 and 630 are controlled by a microprocessor such asmicro-controller 220 (FIG. 2). Essentially, micro-controller 220 cyclesmultiplexer 628 and 630 so that one of the eight input signals isprovided as its output signal according to a pre-determined rotation orsequence.

The outputs from multiplexers 628 and 630 enter amplification stages 632and 638 respectively. In each of the amplification stages 632 and 638,there is provided an inverting and non-inverting amplifier. Inamplification stage 630, there is non-inverting amplifier 634 andinverting amplifier 636. In amplification stage 638, there is anon-inverting amplifier 642 and an inverting amplifier 640. Set up inthis fashion, one of the outputs of amplification stages 632 and 638will always be positive and of sufficient amplitude to trigger at leastone of the threshold detectors 644-650. In the exemplary embodiment ofthe invention, the amplifiers 632-638 are conventional 224 operationalamplifiers.

Threshold detectors 644 and 650 comprise four NPN transistors in theexemplary embodiment of the present invention. The outputs fromamplification stages 632 and 638 drive the base-emitter junction of eachof these four NPN transistors, respectively. The four collectors of thetransistors are wired together in wired OR configuration (the OR gate652) to generate an active low signal that determines the state of theresolver windings. If there are no "opens" in the stator windings of theresolver, one of the four output signals (2 from amplification stage 632and two from amplification stage 638) will be greater than zero voltsand one of the transistors making up threshold detections to thresholddetector 644-650 will pull the output signal "low". The output signal ofthe OR gate 652 is latched by a conventional latch circuit. This latchis then polled by the microcontroller 220 during the next samplinginterval to determine if any of the windings of the selected resolver isexhibiting an open circuit condition.

If the rotor of the resolver is moving, this circuit will detect an opencondition in the rotor winding (primary) or either of the two stator(secondary) windings. If the rotor is not moving, however, this circuitwill only detect an open condition if the primary winding or both of thesecondary windings are open. In an exceptional condition, the circuitwill detect an open condition if only one secondary winding is open andthe rotor is at a quadrant boundary.

This circuit operates as follows, as long as all secondary windings areoperational and the rotor of the resolver is moving, at least one of thesine, cosine, inverted sine or inverted cosine signals will be positiveat all times. Thus, at least one of the threshold detectors 644-650 willprovide a logic-high value to the OR gate 652 and the output signal ofthe OR gate 652 will be logic-high. If, however, the rotor is moving andeither of the secondary windings of the resolvers is open, at some time,all of the sine or cosine output signals for the resolver will be zero.This event forces the output signal of the amplification stages 632through 638 to be below the turn-on voltage of their correspondingthreshold detectors 644-650. In this instance, all four thresholddetectors are off and the open signal becomes logic-low. This causessignal SAMPLE CLOCK to latch a logic-low signal in the latch 654, andthus detect an open circuit condition. When an open circuit is detected,an error condition is flagged and position data from the flaggedresolver is ignored.

The position detector portion of the circuit shown in FIG. 6 operates asfollows. During normal operation, the signals from the resolvers leavelow-pass filter 614 and 616 and enter polarity detector 618 and 620.Polarity detector 618 and 620 are used to determined which quadrant theresolvers are in. The signals from polarity detectors 618 and 620 arelatched in latches 622 and 624 via a clock pulse from micro-controller222. The output data from latches 622 and 624 is then multiplexed in themultiplexor 626 and provided to the microcontroller 220 which uses thedata to determine the angular position of the eight axes and otherfactors, such as, whether motion or a jitter condition has occurred.

FIG. 7 is a block diagram, partly in schematic diagram form of anexcitation signal switch suitable for use as the switch 210. In normaloperation, the external excitation signal from the servo controller 116is passed through two N-channel field effect transistors (N-FETs) 710and 712 to the resolver excitation coils. When, however, the valid servodata detector has detected an invalid servo signal, the micro-controller220 conditions the switch 210 to generate the internal excitation signalhaving trapezoidal pulses by modulating the gate electrode of aP-channel field effect transistor (P-FET) 714.

The micro-controller 220 selects Which of the internal or externalexcitation signals is to be applied to the resolvers 112 using controlsignals RPV and SEV. When RPV is in a logic-high state, the internalexcitation signal is selected and when SEV is in a logic high state, theexternal excitation signal is selected. The micro-controller 220includes logic to ensure that the signals SEV and RPV are never in alogic-high state at the same time.

The signals SEV and RPV are applied to the control input port of atwo-bit four-way multiplexer 716. The data inputs 1A, 1C, 1D, and 2Bthrough 2D are coupled to a source of reference potential (e.g. ground)which acts as a logic-low signal. Data input terminal 2A of themultiplexer 716 is coupled to receive a logic-high signal from a sourceof logic-high signal 718. Data input terminal 1B of the multiplexer iscoupled to receive a signal SHORT provided by a latching COMPARATOR 720.

As set forth below, the signal SHORT is in a logic-low state when ashort circuit condition exists in the line that connects the excitationsignal to the resolvers or in the rotor winding of one of the resolvers.In normal operation, the signal SHORT is in a logic-high state.

The output signals A and B provided by the multiplexer 716 as a functionof the control signals RPV and SEV are shown in Table 2.

                  TABLE 2                                                         ______________________________________                                        RPV      SEV           A        B                                             ______________________________________                                        0        0             0        0                                             0        1             0        1                                             1        0             SHORT    0                                             1        1             0        0                                             ______________________________________                                    

Selecting the signal SHORT to control the excitation signal switch whenthe internal excitation signal is selected inhibits the generation ofthe internal excitation signal when there is a short-circuit conditionin the inputs of the resolvers 112. This protects the RPT 114 fromdamage which may occur if the P-FET 714 were to drive a short circuit.

As shown in FIG. 7, the signals A and B enable respective amplifiers 722and 724. When the internal excitation signal is selected, themicro-controller 220 provides a square-wave pulse signal, PULSE, to theexcitation signal switch 210. The signal PULSE is passed by the gatedamplifier 722 to a switch 728. The switch is responsive to the signalPULSE to activate a controlled current source 732 when PULSE islogic-high and to activate a current source 730 when PULSE is logic-low.The current source 732 drains current from the gate electrode of theP-FET 714 to a voltage source -VP, having a potential less than groundpotential, at a relatively low rate. This relatively low current causesthe conduction channel of the P-FET 714 to exhibit a logarithmicallyvarying decreasing resistance, generating a signal at its drain terminalwhich has a slowly rising, positive-going transition. This type oftransition is desirable to prevent transient distortion such as"ringing" on the excitation signal. This type of distortion may occurbecause the excitation signal is driving a reactive load which has thecharacteristics of a resistor-inductor-capacitor (RLC) circuit.

When the signal PULSE exhibits a negative transition, the switch 728deactivates current source 732 and activates current source 730. Thecurrent source 730 charges the gate capacitance of the P-FET 714relatively rapidly to a potential +VP, causing the conduction channel ofthe P-FET 714 to become non-conductive. This rapid transition may causeringing distortion to occur after the negative-going transition of theexcitation signal. This distortion is of minimal significance, however,since, when the internal excitation signal is being used, only thepositive-going transition of the excitation signal is relevant to theposition samples obtained from the output signals of the resolvers 112.Furthermore, by rapidly turning off P-FET 714, battery power isconserved.

The excitation signal switch shown in FIG. 7 includes a diode 715 whichacts to prevent current flow from the N-type transistors 710 and 712when the external excitation signal is active.

In addition, the excitation signal switch includes a short detectorwhich includes a low-valued (e.g. >1 ohm) resistor 740 that is in serieswith the supply +VP and the source electrode of the P-FET 714. Thisresistor is coupled between the inverting and non-inverting inputterminals of an operational amplifier 742. The amplifier amplifies thevoltage developed across the resistor 740 and applies the result to oneinput terminal of a latching comparator 720. The other input terminal ofthe comparator is coupled to receive a reference potential provided by avoltage divider. If the voltage across the resistor ever exceeds thereference potential, the output signal of the comparator latches at alogic-low value.

The signal SHORT is read by the microcontroller 220 during each sampleinterval. An active short condition (i.e. SHORT=logic-low) causes themicrocontroller 220 to flag an error condition and cease operation ofthe RPT 114.

When the signal B, provided by the multiplexer 716 is logic-high, theamplifier 724 is enabled. This amplifier is coupled to a source ofoperational potential VDRV. The amplified DC potential provided by theamplifier 724 is applied to an isolated voltage source 738. The source738 which may, for example, be a DIG-11-8-30-DD device available fromDionics, Inc. includes a bank of light-emitting diodes (not shown) whichilluminate a photocell (not shown). In response to this illumination,the photocell develops a potential of, for example, 12 volts across thetwo output terminals of the source 738. The negative output terminal(GND) of the source 738 is coupled to the interconnected sourceelectrodes of the N-FETs 710 and 712 while the positive output terminal(VOUT) is connected to the gate electrodes. In this configuration, theN-FETs 712 and 710 are always conductive when the output signal B of themultiplexer 716 is in a logic-high state.

The N-FETs used in the exemplary embodiment of the invention are IRF511devices available from International Rectifier. These devices wereselected for their relatively low channel resistance in the conductivestate. A low resistance is important since it is desirable to pass theexternal sinusoidal excitation signal to the resolvers 112 withoutsignificant distortion components. An undistorted sinusoidal excitationsignal produces the most accurate angular measurements from theresolvers 112.

The excitation signal output terminal of the RPT 114 is shunted toground by a transient suppressor such as a transorb 744. In theexemplary embodiment of the invention, this device is a model no.DO-215AA available from General Semiconductor Industries, Inc. Thistransient suppressor acts as a high impedance when the excitation signalis in the normal range of approximately ±6 volts. If, however, theexcitation signal exceeds this range in either the positive or negativedirection, the transorb 744 exhibits a relatively low impedance,shunting the excess potential to ground. The transorb 744 suppressestransients, such as voltage spikes, which may be caused, for example bynear-by lightning strikes or by unshielded equipment operating nearby.In addition, the transorb 744 at least partially suppresses any ringingtransients that may occur due to the relatively rapid negative-goingtransition of the internal excitation signal.

The RPT 114 uses the magnitude detector circuit 212 (FIG. 2) todetermine when to switch between passing the external sinusoidalexcitation signal and providing the internal trapezoidal excitationsignal to the resolvers 112. As described above, the magnitude detectorcircuit monitors the servo excitation signal to determine if themagnitudes of either the negative or positive polarity portion of thesignal has fallen below a preset threshold value.

FIG. 8 is a block diagram, partly in schematic diagram form of circuitrysuitable for use as the magnitude detector 212. FIG. 8 may be dividedinto two equivalent parts, a positive peak magnitude detector and anegative peak magnitude detector. For the sake of brevity only thepositive peak magnitude detector is described below, followed by a briefdescription of the differences between it and the negative peakmagnitude detector. The positive and negative peak detectors use clocksignals derived from the sinusoidal servo excitation signal. Asdescribed below with reference to FIG. 9, the clock signal SERVO CLK islocked in phase to the sinusoidal excitation signal and haspositive-going transitions substantially coincident with the positivepeaks of the excitation signal. An inverted signal, SERVO CLK* is alsoprovided by an inverter 824. The positive-going transitions of thisclock signal occur substantially coincident with the negative peaks ofthe sinusoidal excitation signal.

In the positive peak magnitude detector, the sinusoidal servo excitationsignal provided by the servo controller 116 is applied to anon-inverting amplifier 810. The amplifier 810 scales the excitationsignal to fit within the dynamic range of a comparator 814. The outputsignal provided by the amplifier 810 is applied to one terminal of acapacitor 812, the other terminal of which is coupled to ground. Inaddition, the output terminal of the amplifier 810 is coupled to oneterminal of a resistor 816, the other terminal of which is coupled toground through a gate 818.

During the positive half of one cycle of excitation signal, the gate 818is open and the output signal of the amplifier charges the capacitor 812to the positive peak of the excitation signal through the resistor 817.As the capacitor is charging, the comparator 814 is comparing thevoltage across the capacitor to a reference voltage provided by avoltage divider circuit 815. If the voltage across the capacitor isgreater than the reference voltage, the output signal of the comparatoris in a logic-high state. Otherwise it is in a logic-low state.

The combination of resistor 817 and capacitor 812 form aresistor-capacitor (RC) circuit having an RC time constant. This circuitlimits the rate at which the capacitor 812 can charge to mitigate anyeffects of power surges on the capacitor 812.

At the peak of the sinusoidal excitation signal, the signal SERVO CLK*exhibits a positive going transition. This signal causes the latch 820to store the output signal of the comparator 814 and, simultaneously,causes the gate 818 to close, allowing the capacitor 812 to discharge toground through the resistors 816 and 817.

In addition to operating the latch 820, the signal SERVO CLK* operates astate machine which compares the value, Q, provided by the latch 820 tothe two previously provided values (Q-1 and Q-2). The output signal,PMAG, provided by the state machine indicates whether the magnitude ofthe positive peaks of the sinusoidal excitation signal is sufficient(PMAG is logic-high) or insufficient (PMAG is logic-low) for threeconsecutive sampling periods. Table 3 illustrates the operation of thestate machine 822.

                  TABLE 3                                                         ______________________________________                                        Q-2      Q-1    Q       Old PMAG New PMAG                                     ______________________________________                                        0        0      0       0        0                                            0        0      0       1        0                                            0        0      1       0        0                                            0        0      1       1        1                                            0        1      0       0        0                                            0        1      0       1        1                                            0        1      1       0        0                                            0        1      1       1        1                                            1        0      0       0        0                                            1        0      0       1        1                                            1        0      1       0        0                                            1        0      1       1        1                                            1        1      0       0        0                                            1        1      0       1        1                                            1        1      1       0        1                                            1        1      1       1        1                                            ______________________________________                                    

The negative peak detector includes inverting amplifier 830, resistor837, capacitor 832, comparator 834, voltage divider 835, dischargeresistor 836, controlled gate 838, latch 840 and state machine 842. Thispeak detector operates in the same manner as the positive peak detectorexcept that the inverting amplifier 830 produces the magnitude of thenegative-going peak of the sinusoidal excitation signal across thecapacitor 832. By substituting the clock signal SERVO CLK for the signalSERVO CLK*, this value is compared to the reference value at the propertime and latched into the latch 840. Thus, the output signal NMAG of thestate machine 842 indicates whether the magnitude of the negativeportion of the sinusoidal excitation signal is sufficient or notsufficient.

The signals PMAG and NMAG are combined by an AND gate 826 to generate asignal MAGDET which is applied to the microcontroller 220. When thissignal is in a logic-high state, the sinusoidal excitation signal isvalid, otherwise it is invalid. The microcontroller polls this signalwhen each sample of the signals provided by the detector circuit istaken. As described above, if MAGDET changes state from logic-high tologic-low, the microcontroller 220 (FIG. 2) conditions the excitationsignal switch 210 (FIG. 2) to provide the internally generatedtrapezoidal excitation signal to the resolvers 112 in place of thesinusoidal excitation signal.

FIGS. 3c and 3d are timing diagrams which illustrate the operation ofRPT 114 as it switches between providing the external excitation signaland providing the internal excitation signal at both fast and slowsample rates.

In FIG. 3c, between times T1 and T2, a valid external excitation signalis present and the RPT 114 samples the position of the resolvers using afast, or high-frequency SAMPLE CLOCK signal. At time T2, the externalsinusoidal excitation signal is detected as invalid, as indicated by thesignal MAGDET falling from logic-high to logic-low, and the RPT 114switches to provide the trapezoidal internal excitation signal. Thesystem is operating in the normal mode, using the high frequency SAMPLECLOCK signal since motion is present at time T2. At time T3, no motionis present and the RPT 114 switches to its low-power operating mode,emitting excitation pulses and taking position samples at a lower rate.This mode is described below with reference to FIG. 5.

At time T4, a valid sinusoidal excitation signal is detected and the RPT114 switches to provide the sinusoidal excitation signal to theresolvers 112 and to sample the resolver position signals using the fastSAMPLE CLOCK signal.

In FIG. 3d, between times T5 and T6, the RPT 114 is in its low-powermode, issuing excitation pulses and sampling the resolver position usingthe slow SAMPLE CLOCK signal. At time T6, motion is detected, causingthe RPT 114 to switch to its normal mode using the fast SAMPLE CLOCKsignal. At time T7, a valid external excitation signal is detected,causing the RPT 114 to switch from providing its internally generatedtrapezoidal excitation signal to passing the external sinusoidalexcitation signal.

During the interval when the sinusoidal excitation signal is failing,for example, immediately prior to time T2, there may be sample intervalswhen the resolvers 112 are subject to an invalid excitation signal. Ifthe servo controller 116 fails gracefully, there should be no invalidresolver samples since excitation signal levels flagged as invalid bythe magnitude detector 212 can still produce valid output signals fromthe resolvers. If the servo controller fails catastrophically, however,going instantly from a valid signal level to no signal, the detectioncircuit 218 may produce position samples based on an invalid excitationsignal. Invalid samples of this type may also occur if the sinusoidalexcitation signal is contaminated with noise signal components.

In the exemplary embodiment of the invention, these invalid samplingintervals are detected by the valid servo data detector 214, shown inFIG. 9. Also shown in FIG. 9 is a block diagram, partly in schematicdiagram form of an exemplary clock generator 216. Before describing thevalid servo data detector, it is helpful to describe the clock generator216.

The clock generator produces the clock signal SERVO CLK which is lockedin phase to the sinusoidal servo excitation signal. In this circuit, thepositive and negative peaks of the external servo excitation signal areclamped to predetermined positive and negative potential values by theaction of a pair of diodes 910. The signal produced by the diodes iscompared, by a comparator 914, to a reference potential (e.g. 0 volts)generated by a voltage divider 912. The comparator 914 acts to invertthe servo excitation signal and to convert it to square-wave signal.This square-wave signal is applied as a reference signal to aconventional phase-locked-loop (PLL) circuit 916. In response to thisreference signal, the PLL 916 produces the clock signal SERVO CLK whichis locked in phase to the sinusoidal reference signal and which exhibitsnegative-going transitions that occur substantially coincident withpositive peaks of the sinusoidal excitation signal. As described above,this signal is applied to the magnitude detector 212 and to the validservo data detector 214.

In addition, the SERVO CLK signal is applied to one input terminal of amultiplexer 918, the other input terminal of which is coupled to receivea signal MP CLOCK which is generated by the microprocessor 220, asdescribed above. The multiplexer 918 is controlled by the signal SEV toprovide the SERVO CLK signal or the MP CLOCK signal as its output signalSAMPLE CLOCK when SEV is logic-high or logic-low, respectively. Asdescribed above, the signal SAMPLE CLOCK is used by the detectioncircuit to determine quadrant information from the output signals of theresolvers 112.

The valid servo data detector 216 includes a diode clamp 920, voltagedivider 922 and comparator 924 which operate in the same manner as thecorresponding components 910, 912 and 914 in the clock generator 214. Inthe valid servo data detector, however, the output signal of thecomparator 924 which indicates whether the positive peaks of the servoexcitation signal exceed a predetermined threshold value is stored intoa latch 928 synchronous with a signal that is delayed, for example,three microseconds by a delay element 926, with respect to the signalSERVO CLK. The valid servo data detector 216 samples the servoexcitation signal at a time delayed with respect to the time that theresolver output data is latched.

The output signal of the latch 928 is polled by the microcontroller 220each time it receives a position sample from the detection circuit 218.If this signal is in a logic-high state then the microcontroller 220stores the sample value. If, however, this signal is in a logic-lowstate, indicating an invalid sample value, the microcontroller 220ignores the sample value. It is better to skip over a possibly invalidsample value when the RPT 114 is operating in synchronism with thesinusoidal servo excitation signal since samples occur with sufficientfrequency so that one or two samples may be skipped without producing aninvalid sample sequence.

Jitter Detection

FIG. 3a shows an example of movement of one of the resolvers 112 throughquadrants Q₀ -Q₃. For purposes of illustration, quadrant position isshown by a small circle in each of the quadrants in the directionindicated by the arrows. Position detection does not necessarily have totake place in the center of each quadrant nor be limited to clockwise orcounter clockwise rotation.

FIG. 3b shows the "jitter" condition. During a jitter condition, theresolver (and, thus the motor) oscillates at the boundary between twoquadrants. This oscillation occurs when the motor of robot 110 stops ator near the boundary between two quadrants.

During a main power failure or shut down, it is desirable to conservepower by switching the RPT 114 into its low-power mode and using anexcitation and sample rate that is slower than when no motion is beingdetected. When motion is detected, it is desirable to use a fastersampling rate so that no position information is lost. A jittercondition has the potential for being detected as motion since theresolver is sensed as moving from one quadrant to another. This couldcause unnecessary power usage by maintaining a fast sample rate whenmotion is not present.

Jitter is prevented by comparing the current position of the resolver tothe last two positions, each time the resolver enters a new quadrant. Ifthe results show an oscillation between adjacent quadrants, a jittercondition in declared and the switch to a faster sampling rate is notmade. A jitter control register (JCR) is established which stores thecurrent, previous and next-most-previous position information of theresolver. The JCR is compared to a look-up table to determine theaddress for the subroutines for determining motion or jitter. The JCRcould also be used in a real time calculation comparing the order of thepositions in the three registers in the JCR. In table 4, below, thestate column indicates the state of motion of the resolver. Column Crepresents the current quadrant; column C₁ represents the last quadrant;C₂ represents the next to last quadrant. All possible combinations areshown, although it is understood that "illegal" and "invalid" conditionswould be filtered out before reaching the look-up table.

                  TABLE 4                                                         ______________________________________                                        STATE    C-2    C-1    C   STATE    C-2  C-1  C                               ______________________________________                                        INITIAL  0      0      0   ILLEGAL  2    0    0                               START.sub.-- UP                                                                        0      0      1   ILLEGAL  2    0    1                               ILLEGAL  0      0      2   ILLEGAL  2    0    2                               START.sub.-- UP                                                                        0      0      3   ILLEGAL  2    0    3                               JITTER   0      1      0   MOTION   2    1    0                               INVALID  0      1      1   INVALID  2    1    1                               MOTION   0      1      2   JITTER   2    1    2                               ILLEGAL  0      1      3   ILLEGAL  2    1    3                               ILLEGAL  0      2      0   ILLEGAL  2    2    0                               ILLEGAL  0      2      1   START.sub.-- UP                                                                        2    2    1                               ILLEGAL  0      2      2   INITIAL  2    2    2                               ILLEGAL  0      2      3   START.sub.-- UP                                                                        2    2    3                               JITTER   0      3      0   MOTION   2    3    0                               ILLEGAL  0      3      1   ILLEGAL  2    3    1                               MOTION   0      3      2   JITTER   2    3    2                               INVALID  0      3      3   INVALID  2    3    3                               INVALID  1      0      0   INVALID  3    0    0                               JITTER   1      0      1   MOTION   3    0    1                               ILLEGAL  1      0      2   ILLEGAL  3    0    2                               MOTION   1      0      3   JITTER   3    0    3                               START.sub.-- UP                                                                        1      1      0   ILLEGAL  3    1    0                               INITIAL  1      1      1   ILLEGAL  3    1    1                               START.sub.-- UP                                                                        1      1      2   ILLEGAL  3    1    2                               ILLEGAL  1      1      3   ILLEGAL  3    1    3                               ILLEGAL  1      2      0   ILLEGAL  3    2    0                               JITTER   1      2      1   MOTION   3    2    1                               INVALID  1      2      2   INVALID  3    2    2                               MOTION   1      2      3   JITTER   3    2    3                               ILLEGAL  1      3      0   START.sub.-- UP                                                                        3    3    0                               ILLEGAL  1      3      1   ILLEGAL  3    3    1                               ILLEGAL  1      3      2   START-UP 3    3    2                               ILLEGAL  1      3      3   INITIAL  3    3    3                               ______________________________________                                    

There is shown in FIG. 4 a flow diagram showing the jitter detectionprocess. The jitter control register (JCR) is initialized in block 410.Three copies of initial data are stored in the three registers and thisis considered the INITIAL state. Next, it is determined whether a firstaxis is crossed in decision block 412. The first axis crossing causesthe JCR to be in the START-UP state is accepted as motion and theposition tracker should be switched to enter the fast sampling ratemode. The JCR is updated to the START-UP state in block 414. Whether asubsequent axis is crossed is determined in decision block 416. Thesecond or subsequent axis crossing causes the JCR to be neither thejitter nor motion states. If no axis is crossed, processing loops backuntil an axis crossing is detected. If an axis crossing is detected, theJCR is updated in block 418 and the JCR must be evaluated in decisionblock 420 to determine whether the axis crossing is jitter or motion. Ifthe axis crossing is determined to be jitter, the system remains in theslow sample mode as shown in block 420 and processing loops back todetermine whether a subsequent axis is crossed in block 416. If motionis shown by the JCR, the system is switched to the fast sample mode inblock 424. Following the switching to fast sample mode 424, it isdetermined whether motion has stopped in decision block 426. When motionhas stopped, the system is switched back to slow sampling mode and block512, the transition time through a quadrant (such as quadrant Q0 shownin FIG. 3a) is determined. The transition time determined in block 512is actually the minimum transition time or the fastest time in which theresolver (and, thus the particular part of the robot device 110) canmove through any quadrant. In block 514, it is determined what thesample time is at the fast sample rate. Information determined in blocks510, 512 and 514 can be set up as initialized constant values orvariable values that are updated as conditions for the system change. Anexample would be a change in data rates as performance of the robot isenhanced or diminishes depending on the particular application.

In block 516, it is determined whether the sample time of the fastsample rate is greater than the transition time of the resolver througha quadrant (such as quadrant Q₀). To effectively sample, it is necessarythat the fast sample rate have a resolution sufficient to detect motion.This requires that the sample rate be faster than the fastest possiblemotion through a given quadrant. Similarly, in block 518, it isdetermined whether the transition time through a quadrant is greaterthan the sleep time plus the processing time of the microprocessor. Ifthe transition time through a quadrant is less than the sleep time plusthe processing time, the slow sampling rate will be unable to detectmotion as it exceeds the time necessary to pass through a processingloops back to decision block 416 to determine whether a subsequent axishas been crossed.

The purpose of using the current, previous and next-most-previousquadrants is to determine whether the resolver has passed through threesequential quadrants, indicating that true motion has occurred. If theJCR contains quadrant information which does not indicate that theresolver has traversed three sequential quadrants but instead has passedfrom a first quadrant to a second and the back to the first, a jittercondition is evident. Other entries in the JCR Table 4 consist ofillegal or invalid conditions. These entries are in the table as"filler" and are not valid states for the resolver but are merely shownto illustrate all possible combinations of the JCR.

In the present system, the JCR is implemented in the RAM of themicrocontroller 220 (shown in FIG. 2). The look-up table shown in Table4, is implemented in EEPROM 228 (shown in FIG. 2).

FIG. 5 is a flow diagram for optimizing the data sampling in the presentinvention. Because of the concern with conserving power during aninterruption of the main power supply, it is desirable to have the mostefficient system for switching between the fast sampling rate and theslow sampling rate. The fast sampling rate consumes more power than theslow sampling rate and should thus be used sparingly (i.e. when motionis detected and continuing). Since the fast and slow sampling rates areessentially clock pulses, the most efficient implementation of thesystem determines the minimum number of fast data samples requiredbefore the data sampling rate can be decreased to the slow samplingrate. As will be apparent to those skilled in the art, this is equallytrue in any other system that can change its data periods at a fixedacceleration rate. In order to effectuate such a system, it is necessaryto make certain determinations before the data sampling rate can beoptimized for maximum power conservation. In the exemplary embodiment,these determinations made off-line and are input to the RPT 114 asconstants.

Referring back to FIG. 5, it is first determined in block 510 what thesleep time and processing time is for the microcontroller 220. The sleeptime is the amount of time in which the processor does not have to beactively sampling and interpreting the resolver position data to detectmotion. Having a long sleep time would save power since themicrocontroller is performing only minimal functions directed at motiondetection during this time. The processing time is the amount of timenecessary for the microprocessor to read and interpret the return signalfrom the excitation by the trapezoidal pulse signal. In quadrant.Accordingly, in such a situation, the resolver would be able to passcompletely through quadrant before motion would be detected in thesubsequent quadrant. If either condition in block 516 or 518 isnegative, an error condition results in block 520.

It is possible, however, that if block 518 results in the transitiontime being less than the sleep time plus the processing time, that theprocessor will merely maintain the fast sampling rate and not reset tothe slow sampling rate. In such an operation, the negative outcome of518 is not necessarily an error condition, but possibly an acceptablecondition whereby the fast sampling rate is permanently maintained. Asthis system is used to optimize the data sampling rate's selection, itis more desirable to have access to a slower sampling rate. If motion isdetected in block 522, a fast sampling rate counter is initialized inblock 524 and counting begins in block 526. If motion is again detectedin block 528, the fast sample rate counter is reinitialized in block 524and loops back through block 526 to 528 until no motion is detected.

When no motion is detected, a comparison is done between the fast samplerate counter and the sleep time plus processing time in block 530. Thiscomparison is actually a comparison of the amount of time taken up bythe fast sampling rate. The fast sampling rate counter when multipliedby the period of the fast sampling rate results in the time expended infast sampling. When this time is greater than the sleep and theprocessing time of the microprocessor, it is possible to sample at theslower rate. The switch to the slow sample rate is accomplished in block532.

Another way to visualize the logic of FIG. 5 is to determine the numberof fast sample rates which make up a slow sample rate. After motion hasstopped and the number of fast sample rates detected exceed the periodof the slow sample rate, effective monitoring can be done at the slowsample rate. For best operation, however, the count of fast sample rateperiods is desirably reinitialized each time motion is detected. Thisassures that there is not motion and thus no need for the fast samplingrate. At this point, it is safe to switch to the slow sampling rate toconserve power.

The invention has been described in terms of an exemplary embodiment. Itis contemplated, however, that it may be practiced with modifications asoutlined above within the scope of the appended claims.

What is claimed:
 1. In a servomotor control system including a resolverintegrally coupled to a servomotor and responsive to an excitationsignal to provide an indication of the instantaneous angular position ofthe servomotor, apparatus comprising:a source of external excitationsignal; a source of pulse signal having a predetermined frequency; firstcontrollable switching means, coupled to receive the external excitationsignal and responsive to a first control signal for selectively passingsaid external excitation signal to the resolver; second controllableswitching means, coupled to receive said pulse signal and responsive toa second control signal, for selectively generating, from said pulsesignal, an internal signal suitable for use as an excitation signal andfor selectively applying the internal signal to the resolver; andcontrol means responsive to a third control signal for generating thefirst and second control signals so that only one of the externalexcitation signal and the internal signal is applied to the resolver atany instant.
 2. In a servomotor control system including a resolverintegrally coupled to a servomotor and responsive to an excitationsignal to provide an indication of the instantaneous angular position ofthe servomotor, apparatus comprising:a source of external excitationsignal; a source of pulse signal having a predetermined frequency; firstcontrollable switching means comprising first and second N-channel fieldeffect transistors (N-FETs), coupled to receive the external excitationsignal and responsive to a first control signal for selectively passingsaid external excitation signal to the resolver; second controllableswitching means, coupled to receive said pulse signal and responsive toa second control signal, for selectively generating, from said pulsesignal, an internal signal suitable for use as an excitation signal andfor selectively applying the internal signal to the resolver; andcontrol means responsive to a third control signal for generating thefirst and second control signals so that only one of the externalexcitation signal and the internal signal is applied to the resolver atany instant.
 3. The servomotor control system of claim 2 wherein saidfirst controllable switching means further comprises a voltage sourceconnected to said first and second N-FETs and adapted to apply a controlvoltage to the N-FETs to maintain the N-FETs in a conductive state whensaid external signal is to be applied to the resolver.
 4. The servomotorcontrol system of claim 3 wherein said voltage source comprises aplurality of light emitting diodes, said light emitting diodesilluminating at least one photocell to produce said control voltage tobe applied to said first and second N-FETs by said voltage source.
 5. Ina servomotor control system including a resolver integrally coupled to aservomotor and responsive to an excitation signal to provide anindication of the instantaneous angular position of the servomotor,apparatus comprising:a source of external excitation signal; a source ofpulse signal having a predetermined frequency; first controllableswitching means, coupled to receive the external excitation signal andresponsive to a first control signal for selectively passing saidexternal excitation signal to the resolver; second controllableswitching means comprising a P-channel field effect transistor (P-FET),coupled to receive said pulse signal and responsive to a second controlsignal, for selectively generating, from said pulse signal, an internalsignal and for selectively applying the internal signal to the resolveras said internal excitation signal; and control means responsive to athird control signal for generating the first and second control signalsso that only one of the external excitation signal and the internalsignal is applied to the resolver at any instant.
 6. The servomotorcontrol system of claim 5 wherein said internal signal is a trapezoidalshaped pulse signal.
 7. The servomotor control system of claim 5 whereinsaid P-FET includes a conductive channel and a gate electrode whichcontrols the conductive channel, and wherein said second controllableswitching means further comprises switchably selected first and secondcurrent sources, said first current source adapted to apply electriccharge to said gate electrode at a first predetermined rate to conditionsaid P-FET to increase its conductivity and said second current sourceadapted to drain electric charge from said gate electrode at a secondpredetermined rate to condition said P-FET to decrease its conductivity,wherein said second predetermined rate is grater than said firstpredetermined rate.
 8. In a servomotor control system including aresolver integrally coupled to a servomotor and responsive to anexcitation signal to provide an indication of the instantaneous angularposition of the servomotor, apparatus comprising:a source of externalexcitation signal; a source of pulse signal having a predeterminedfrequency; first controllable switching means, coupled to receive theexternal excitation signal and responsive to a first control signal forselectively passing said external excitation signal to the resolver;second controllable switching means, coupled to receive said pulsesignal and responsive to a second control signal, for selectivelygenerating, from said pulse signal, an internal signal suitable for useas an excitation signal and for selectively applying the internal signalto the resolver; and control means comprising a multiplexer responsiveto a third control signal for generating the first and second controlsignals so that only one of the external excitation signal and theinternal signal is applied to the resolver at any instant, said thirdcontrol signal comprising first and second switching signals, each ofsaid first and second switching signals having logic high and logic lowstates, said first and second switching signals selectively generated tonot be in the same logic state at the same time.
 9. The servomotorcontrol system of claim 8 further comprising a microcontroller whichgenerates the first and second switching signals.
 10. The servomotorcontrol system of claim 9 wherein said external excitation signal issinusoidal of a predetermined amplitude.
 11. The servomotor controlsystem of claim 10 further comprising a magnitude detector to determinewhether said external excitation signal has fallen below saidpredetermined amplitude, said microcontroller being coupled to saidmagnitude detector to generate said first and second switching signals.